The feast was introduced to complement the celebration of St. Michael on September 29.
Why Catholics celebrate Guardian Angels on October 2? In the current liturgical calendar, there exists two feasts that honor angels. The first is the feast of Sts. Michael, Gabriel and Raphael, celebrated on September 29. Initially, it celebrated only St. Michael and it was also a day of obligation.
Eventually there arose a request to have a separate feast in honor of the guardian angels, those angelic spirits that are assigned by God to guide and protect each human soul.
According to the Catholic Encyclopedia, October 2 was picked because it was the first “free” day in the calendar.
At the time it was chosen, September 30 was the celebration of St. Jerome and October 1 was the commemoration of St. Remigius, patron saint of France. October 2 did not have any particular celebrations and so it became the feast of the Holy Guardian Angels.